Use of Oxygen Gas Diffusion Cathode in Industrial Electrolysis
Use of an oxygen gas diffusion electrode in industrial electrolysis has recently come to be investigated. For example, a hydrophobic cathode for conducting an oxygen reduction reaction is used in an apparatus for the electrolytic production of hydrogen peroxide. Also, in processes for alkali production or acid/alkali recovery, a hydrogen oxidation reaction (hydrogen anode) as a substitute for oxygen generation on an anode or an oxygen reduction reaction (oxygen cathode) as a substitute for hydrogen generation on a cathode is conducted by using a gas diffusion electrode, thereby attaining a reduction in the electric power consumption. It has been reported that when a hydrogen anode is used as a counter electrode in metal recovery, for example, zinc collection or zinc plating, depolarization is possible.
Caustic soda (sodium hydroxide) and chlorine which are important as an industrial raw material are being produced mainly by a sodium chloride electrolysis method. This electrolysis method has shifted through a mercury method in which a mercury cathode is used and the diaphragm method in which an asbestos diaphragm and a soft-iron cathode are used to an ion exchange membrane method in which an ion exchange membrane is used as a diaphragm and an active cathode having a low overvoltage is used. During this interval, the electric power consumption rate required for the production of 1 ton of caustic soda has decreased to 2,000 kWh. However, since the caustic soda production is a large electric consumption industry, a further reduction in the electric power consumption rate is demanded.
In a related-art sodium chloride electrolysis method, an anode reaction and a cathode reaction are shown in the following schemes (1) and (2), respectively, and a theoretical decomposition voltage thereof is 2.19 V.2Cl−-−*C12+2e(1.36 V)  (1)2H2O+2e-+20H−H2(−0.83 V)  (2)
When an oxygen cathode is used in place of conducting a hydrogen generation reaction on a cathode, a reaction shown in the following scheme (3) takes place. As a result, a cell voltage can be reduced theoretically by 1.23 V, or by about 0.8 V even in a practically useful current density range. Thus, a reduction in the electric power consumption rate of 700 kWh per ton of sodium hydroxide can be expected.02+2H20+4e-+40H−(0.40 V)  (3)
For that reason, practical implementation on a sodium chloride electrolysis method utilizing a gas diffusion cathode has been investigated since the 1980s. However, in order to realize this process, it is indispensable to develop an oxygen cathode which is required to have not only high performance but sufficient stability in the electrolysis system.
An oxygen gas cathode in the sodium chloride electrolysis is described in detail in “Domestic/overseas Situation Concerning Oxygen Cathodes for Sodium Chloride Electrolysis” in Soda & Chlorine, Vol. 45, 85 (1994).
Gas Diffusion Cathode for Sodium Chloride Electrolysis
An electrolytic cell of the sodium chloride electrolysis method using an oxygen cathode which is most generally conducted at present is of a type in which an oxygen cathode is disposed on a cathode side of a cation exchange membrane via a cathode chamber (caustic chamber) and oxygen as a raw material is supplied from a gas chamber disposed at the back of the cathode. This cell is configured of three chambers of an anode chamber, a catholyte chamber and a cathode gas chamber and hence, is called a three-chamber type electrolytic cell. The oxygen supplied to the gas chamber diffuses within the electrode and reacts with water in a catalyst layer to form sodium hydroxide. Accordingly, the cathode which is used in this electrolysis method must be a gas diffusion cathode of a so-called gas/liquid separation type through which only oxygen sufficiently permeates and in which a sodium hydroxide solution does not leak out to the gas chamber. A gas diffusion cathode in which a catalyst such as silver and platinum is supported on an electrode substrate obtained by mixing a carbon powder and PTFE and forming the mixture in a sheet form has been proposed as an electrode satisfying those requirements.
However, this type of electrolysis method involves some problems. The carbon powder used as an electrode material is readily deteriorated at high temperatures under the coexistence of sodium hydroxide and oxygen, thereby remarkably lowering the electrode performance. Also, it is difficult to prevent the leakage of the sodium hydroxide solution to the gas chamber side as generated with an increase of liquid pressure and deterioration of the electrode especially in a largesized electrolytic cell.
For the purpose of solving these problems, a novel electrolytic cell has been proposed. This electrolytic cell is characterized in that an oxygen cathode is disposed in intimate contact with an ion exchange membrane (zero gap structure) and that oxygen and water as raw materials are supplied from the back of the electrode, whereas sodium hydroxide as a product is recovered from the back of the electrode or a lower part of the electrode. When this electrolytic cell is used, the problem regarding the foregoing leakage of sodium hydroxide is solved, and the separation between a cathode chamber (caustic chamber) and a gas chamber is not necessary. Since this electrolytic cell is configured of two chambers of a single chamber functioning as both a gas chamber and a cathode chamber (caustic chamber) and an anode chamber, it is called a two-chamber type electrolytic cell.
The performance required for the oxygen cathode which is suitable for an electrolysis process using this electrolytic cell is largely different from that required for related-art oxygen cathodes. Since the sodium hydroxide solution which has leaked out to the back of the electrode is recovered, the electrode need not have a function to separate a caustic chamber from a gas chamber and is not required to have an integrated structure, and size enlargement is relatively easy.
Even when the gas diffusion cathode is used, the formed sodium hydroxide not only moves to the back side but moves in a height direction due to gravity. Accordingly, there is a problem that when the formed sodium hydroxide is in excess, the sodium hydroxide solution resides in the inside of the electrode, thereby inhibiting gas supply. The gas diffusion cathode is required to simultaneously have sufficient gas permeability, sufficient hydrophobicity for avoiding wetting due to a sodium hydroxide solution, and hydrophilicity for enabling a sodium hydroxide solution to readily permeate through the electrode. In order to meet these requirements, a method for disposing a hydrophilic layer between an ion exchange membrane and an electrode is proposed in Japanese Patent No. 3553775.
As an electrolytic cell which is positioned intermediate between these electrolytic cells, an electrolytic cell of a liquid dropping type in which a gas cathode having gas/liquid permeability is disposed slightly apart from a membrane and an alkaline solution is allowed to flow from an upper part thereof through a gap therebetween has also been developed (see U.S. Pat. No. 4,486,276).
Apart from improvements in electrolytic cells, extensive and intensive investigations regarding electrode catalysts and substrates are also being advanced.
JP-A-11-246986 discloses a gas diffusion cathode in which a reaction layer having at least a hydrophilic fine particle and a catalyst fine particle of silver in a mixed state and formed by hot pressing together with a fluorocarbon resin and a gas supply layer are superimposed.
JP-A-2004-149867 discloses a gas diffusion electrode in which a gas diffusion electrode forming fine particle is made of a fluorocarbon resin fine particle, a carbon black fine particle and one or two or more kinds of fine particles selected from a polymeric electrolyte fine particle, a metal colloid, a metal fine particle and a metal oxide fine particle.
JP-A-2004-197130 and JP-A-2004-209468 disclose a gas diffusion cathode for sodium chloride electrolysis using an electrode catalyst which is made of a conductive carrier and a mixture containing a noble metal fine particle and a fine particle of at least one alkaline earth metal or rare earth oxide supported on the conductive carrier.
JP-A-2005-063713 discloses an electrode catalyst which is made of a carbonaceous carrier, a fine particle of a noble metal such as platinum, palladium, iridium, ruthenium and alloys thereof supported on a surface of the carbonaceous carrier, and a surface layer for making the surface of the carbonaceous carrier electrochemically inactive.
JP-A-11-124698 discloses that it is desirable to form a catalyst layer on a surface of an electrode support; that a metal such as platinum, palladium, ruthenium, iridium, copper, cobalt, silver and lead or oxides thereof can be used as the catalyst; and that by mixing such a catalyst with a binder such as fluorocarbon resins as a powder and a solvent such as naphtha to form a paste and adhering it, or applying a salt solution of a catalyst metal on the surface of the support and baking it, or subjecting the salt solution to electroplating or electroless plating by using a reducing agent to form a reaction layer, this reaction layer and a gas supply layer are superimposed to form a gas diffusion electrode.
However, in comparison with fuel cells, since an industrial electrolysis system is severe with respect to operation conditions, it involves a problem that sufficient life and performance of a gas diffusion cathode are not obtained. In particular, there is a problem regarding an increase of overvoltage and a reduction of conductivity due to a reduction of catalytic performance. Concretely, though silver catalysts or carbon particles are mainly utilized at present from the viewpoints of performance and economy, it is known that in electrolysis and electrolysis termination operations, agglomeration or dropping of the particles advances, leading to a cause of the performance reduction. Even in the foregoing known technologies, this problem remains unsolved.